R300 & auto negotiation

Dave Ewart davee at ceu.ox.ac.uk
Fri Jul 23 04:05:32 CDT 2010


On Friday, 23.07.2010 at 09:48 +0200, Robert von Bismarck wrote:

> From my experience, when you want to be certain, you must force 100mb
> networks to 100-full. Auto-neg is mostly hit'n miss, especially on
> high-end manageable switches like cisco, extreme networks or HP.
> Cheaper linksys or even trendnet (yuk!) usually get it right...

Seriously?  I've never seen the kind of behaviour you describe, I'm
surprised.

One thing to remember is that for auto-negotiation to work reliably,
BOTH devices must be set to auto-negotiate.  Setting speed or duplex on
one end won't work.  So, leave all your switch ports to auto-negotiate
(usually the default anyway) and this should be fine.

A good example of how you can get strange results by messing around with
speed and duplex settings is as follows:

Device A connects to Switch B

Device A and has 'speed 100Mb, full duplex' set on its NIC, switch B is
set to auto-negotiate.  In this situation, the switch will actually end
up with 'speed 100Mb, *HALF* duplex' since many switches can detect
hardcoded speeds but not the duplex; they 'play it safe' on duplex.  So,
this link has 100-Full at one end and 100-Half at the other: a duplex
mismatch.

Fixing the above normally requires one to remove any hardcoded speed or
duplex settings, just let it auto-negotiate.

> I've seen this behavior with plenty of equipments running 100mb, like
> linux or windows servers, especially on sun/solaris servers and even
> cisco routers. At gig speeds auto-neg is mandatory and works well.

Agreed :-)

Dave.

-- 
Dave Ewart
davee at ceu.ox.ac.uk
Computing Manager, Cancer Epidemiology Unit
University of Oxford / Cancer Research UK
N 51.7516, W 1.2152
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